In recent years, multi-scan display apparatus each capable of dealing with various horizontal deflection frequencies are becoming prevalent with the intention of enhancing the user's operationability and the efficiency of production lines.
Horizontal deflection circuits of these multi-scan display apparatus undergo power voltage control or the like depending on the horizontal deflection frequency so as to be compatible with various horizontal deflection frequencies. In the control scheme, the horizontal S-shape correction circuit for absorbing the difference between the curvature of horizontal deflection and the curvature of the horizontal display plane is designed to provide the multi-scan ability through the switching of a plurality of S-shape capacitors in response to the horizontal deflection frequency, in general. Such technique is described in Japanese Patent Application Laid-Open No. 4-74065.
Another proposed technique is a continuous S-shape correction circuit based on two saturable reactors connected in series to the horizontal deflector coil (the continuous S-shape correction mentioned here is the S-shape correction which can take place at a constant accuracy against changes in the horizontal deflection frequency). Such technique is described in Japanese Patent Application Laid-Open No. 62-31269.
A further proposed technique is described in U.S. Pat. No. 4,645,985 wherein an automatic S-correction circuit for a deflection circuit includes an active source of generally sawtooth wave current having an amplitude that is proportional to the deflection frequency.
However, among the above-mentioned prior art arrangements, the scheme of switching S-shape capacitors makes step variations of capacitance, resulting in a large correction error for some horizontal deflection frequency range, i.e., a large linearity error which varies. It is necessary to increase the number of steps for the reduction of the correction error, resulting in an increased circuit scale and circuit cost.
The scheme based on saturable reactors causes the horizontal deflection circuit to consume increased power due to the distribution of the power voltage to the saturable reactors, resulting in a degraded efficiency of the horizontal deflection circuit. Moreover, while this scheme provides a smaller linearity error which is a constant value, such value is about 5% because the characteristics of the saturatable reactors differ from one another and from the ideal characteristics.
As to the scheme when the amplitude of the sawtooth current is proportional to the deflection frequency, here again a variable linearity error results.